1. Field of the Invention
The present invention relates to an inspection system and an inspection method, and more particularly to an inspection system and an inspection method for detecting defects of the pattern formed on an object to be inspected, such as a mask.
2. Background Art
In recent years, as the levels of integration and capacity of large scale integrated circuits (LSIs) increase, there has been a need to continue to reduce the width of the circuit patterns of semiconductor devices. Semiconductor devices are manufactured by a reduced projection exposure apparatus called a “stepper” using original artwork patterns with a circuit pattern formed thereon, that is, masks or reticles (hereinafter referred to collectively as masks). Specifically, the pattern on a mask is transferred to the wafer by exposure to light, thereby forming circuits on the wafer. Masks used to transfer such fine circuit patterns to the wafer are manufactured by electron beam writing apparatuses, which can write micropatterns. Further, effort has been made to develop a laser beam writing apparatus, which uses a laser beam for writing. It should be noted that electron beam apparatuses are also used to directly write a circuit pattern on a wafer.
Incidentally, since the cost to manufacture LSIs is very high, the improvement of the yield is required to make the manufacture economically feasible. However, the dimensions of the patterns for LSI devices, as typified by 1-gigabit class DRAMs (random access memories), are about to be scaled down from the order of submicrons to the order of nanometers. A major cause of loss in yield is due to defects of a mask pattern. Further, since there has been a decrease in the dimensions of LSI patterns formed on semiconductor wafers, the size of pattern defects to be detected is very small. Therefore, high inspection accuracy is required of mask inspection systems for detecting defects of transfer masks used in LSI manufacture.
There are two known mask defect detecting methods: the die-to-die inspection method and the die-to-database inspection method. The die-to-die inspection method is used when the mask to be inspected has thereon a plurality of identical chip patterns, or a plurality of chip patterns each including an identical pattern segment. In this method, these identical chip patterns or identical pattern segments, which are located on the same mask, are compared to each other. This method permits accurate inspection using a relatively simple system configuration, since patterns on the same mask are directly compared to each other. However, this method cannot detect a defect common to both compared patterns. In the die-to-database inspection method, on the other hand, an actual pattern on a mask is compared to reference data generated from the design pattern data that was used to manufacture the mask. Thus, this method allows exact comparison of the pattern with the design pattern data, although the required system size is large since the method requires a processing system for generating a reference image. There is no choice but to use this inspection method when the mask to be inspected has only one chip pattern to be transferred to the wafer.
In die-to-die inspection system, light is emitted from a light source, and the mask to be inspected is irradiated with this light through an optical system. The mask is mounted on a table, and this table is moved so that the emitted beam of light scans the surface of the mask. Light transmitted through or reflected from the mask pass through the lens and are received by image sensors via a lens, thereby forming an image thereon. The optical image thus formed on the image sensor is sent to a comparing unit as the acquisition data. The comparing unit compares the acquisition data with reference data in accordance with an appropriate algorithm, and if they are not identical, the mask is determined to have a defect (see, e.g., Japanese Laid-Open Patent Publication No. 2008-112178).
If the inspection system determines that the mask has a defect, the system stores the optical image and the corresponding reference image on which the detection of the defect is based, together with the coordinates of the defect. Upon completion of the inspection of the mask, the operator visually checks the pattern in which each defect was detected using the observing optical system in the inspection system. Alternatively, the optical image and the reference image of each defect stored in the inspection system may be displayed on the display of the computer so that the operator can easily check the pattern. The operator then determines whether each defect requires a repair and sends to the repair system the mask and information necessary to repair the defects in need of repair. For example the necessary repair information regarding each defect includes: its coordinates in the mask; an indication of whether it is a extrusion defect (i.e., opaque defect), which must be removed from the light shielding film, or a intrusion defect (i.e., clear defect), which must be filled in; and extracted pattern data for recognizing the pattern portion to be repaired by the repair system. The pattern data may be derived from the above optical image stored in the inspection system.
The repair system then repairs the mask in accordance with the information received from the inspection system, that is, for example, the repair system burns off each extrusion defect from the light shielding film by a beam of light such as a focused ion beam (FIB) and fills each intrusion defect by deposition of carbon (see, e.g., Japanese Laid-Open Patent Publication No. 9-63944 (1997)). The repaired mask is then reinspected. Only repaired masks which have successfully passed the reinspection are shipped.
As pattern dimensions have been scaled down, new types of mask defects have been encountered. That is, current problems to be addressed include not only pattern edge irregularities, formation of isolated holes, and adhesion of material, but also subtle pattern line width errors and pattern displacement. A deviation in the pattern line width or in the spacing between adjacent patterns results in a change in the pattern's impedance and hence a change in the performance of the completed LSI. Such subtle defects may be detected by the mask inspection system using an improved defect detection algorithm. However, the repair information conventionally used to repair detected defects is not sufficient to allow the repair system to locate subtle pattern defects without difficulty even if they are detected by the inspection system. Therefore, there is an urgent need to improve the content of repair information.
The present invention has been made in view of the above problems. That is, since conventional repair information includes only optical images captured by the inspection system and coordinate data of defects, the repair system may not be able to locate pattern portions, or defects, to be repaired if they have shapes typical of recent fine patterns, which shapes are hard to identify. To overcome this problem, it is therefore an object of the present invention to provide an inspection system having a function to output information that allows easy identification of pattern portions to be repaired. Another object of the present invention is to provide an inspection method capable of improving yield and throughput of the entire manufacturing process by receiving information that allows easy identification of pattern portions to be repaired.
Other challenges and advantages of the present invention are apparent from the following description.